1. Field of the Invention
The invention in general relates to miniature switches, and more particularly, to a MEMS switch useful in radar and other microwave applications.
2. Description of Related Art
A variety of MEMS (microelectromechanical systems) switches are in use, or proposed for use, in radar and communication systems, as well as other high frequency circuits for controlling RF signals. These MEMS switches are popular insofar as they can have a relatively high off impedance, with a low off capacitance, and a relatively low on impedance, with a high on capacitance, leading to desirable high cutoff frequencies and wide bandwidth operation. Additionally, the MEMS switches have a small footprint, can operate at high RF voltages and are compatible with conventional integrated circuit fabrication techniques.
Many of these MEMS switches generally have electrostatic elements, such as opposed electrodes, which are attracted to one another upon application of a DC pull down control voltage. At least one of these DC pull down electrodes is on a substrate and an opposing electrode is defined on the underside of a moveable bridge above the substrate. Upon application of the DC pull down control voltage, the bridge is deflected down and the electrical impedance is severely reduced (either by capacitive coupling or by direct ohmic contact), between first and second spaced apart RF conductors on the substrate.
In some MEMS switches the particular bridge design creates asymmetric transverse and longitudinal vibration modes during operation. Switching between on and off states moves the bridge and excites vibration modes, which can lead to undesirable electrical impedance modulation. This impedance modulation is further increased with bridge designs that are laterally asymmetric, causing twisting modes to occur.
In addition, the bridge may be fabricated from different layers. Internal stresses in the bridge""s arms can cause the bridge arms to curl and thereby stiffen. This stiffening due to stress-induced curling can increase the pull down voltage requirement by more than 100%. This is undesirable from an integrated circuit operating point of view.
In conventional capacitive type MEMS switches the portion of the conductor below the bridge is covered with a dielectric layer. Repeated application of the DC pull down voltage between the bridge and pull down conductor causes a charge build up in the dielectric. This charge build up in the dielectric may cause the bridge to stick and remain attracted to the conductor in an on condition, even after removal of the pull down voltage.
It is an object of the present invention to provide a MEMS switch which reduces or eliminates the undesired asymmetric transverse and longitudinal vibration modes in the bridge structure.
It is a further object to eliminate the sticking problem in a capacitive type MEMS switch produced by charge built up in the dielectric.
A MEMS switch is provided which includes a substrate member with first and second spaced-apart RF conductors deposited on the substrate. A bridge member having at least three radially disposed arms of equal length is connected to a support arrangement on the substrate, with each arm having one end connected to the support arrangement and a second end integral with a common central bridge portion having an undersurface. At least one of the arms is electrically connected to the second conductor. The first conductor has an end portion facing the undersurface of the central bridge portion, with the end portion of the first conductor being constructed and arranged to define an open area. A pull down electrode is disposed within the open area of the first conductor and is electrically isolated from the conductor. The height of the pull down electrode is less than that of the end portion. The central bridge portion is drawn toward the first conductor upon application of a control voltage to the pull down electrode, to vary the electrical impedance between the first and second conductors. The impedance is varied from a high value (off state) to a low value (on state) relative to the impedance of the conductors, thus allowing a signal to propagate between the first and second conductors.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood, however, that the detailed description and specific example, while disclosing the preferred embodiment of the invention, is provided by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art, from the detailed description.